This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device having a wide viewing angle by carrying out display with molecule axes in liquid crystal molecules rotated in a plane which is substantially parallel to a surface of a substrate.
One of liquid crystal display device is a color TFT type liquid crystal display device in which thin film transistors (TFTs) are used as switching elements and which is capable of carrying out a color display.
A conventional liquid crystal display device is disclosed in Japanese Unexamined Patent Publication No. Hei 6-160878, namely, JP-A 6-160878. That is, JP-A 6-160878 obtains an active matrix type liquid crystal display device having such features that the contrast is high, visual angle property is good, a multi-level display can be easily performed, and that the display is bright and the cost is reduced. The active matrix type liquid crystal display device has such a structure that an electric field parallel to a substrate surface is impressed on a liquid crystal composition layer by a thin film transistor provided with a drain electrode and a common electrode which are extending over a plurality of picture elements and a source electrode extending in a direction same as that of the drain and the common electrodes.
In the manner which will later be described in conjunction with FIGS. 1A, 1B, 2A, and 2B in more detail, the conventional liquid crystal display device comprises first and second glass substrates spaced apart in a longitudinal direction. The first and the second glass substrates are substantially flat in shape. The first glass substrate has a first principal surface while the second glass substrate has a second principal surface opposed to the first principal surface via a space. In the manner known in the art, a plurality of thin film transistors (TFTs) are formed on the first principal surface of the first glass substrate in an array or matrix fashion. On the other hands, a plurality of color filters (CFs) are formed on the second principal surface in the manner known in the art.
The first glass substrate has a first back surface opposed to the first principal surface. A first polarizer is attached to the first back surface of the first glass substrate. Likewise, the second glass substrate has a second back surface opposed to the second principal surface. A second polarizer is attached to the second back surface of the second glass substrate. The first and the second polarizers may have first and second light transmission axes, respectively, which are perpendicular to each other.
A plurality of pixel electrodes are formed on the first principal surface of the first glass substrate in an array or matrix fashion. The pixel electrodes are equal in number to the thin film transistors and are called display electrodes. Each pixel electrode corresponds to a pixel and extends in a vertical direction or a panel up-and-down direction. Similarly, a plurality of common electrodes are formed on the second principal surface of the second glass substrate in the manner known in the art. Each common electrode extends in the vertical direction with its position shifted from that of the pixel electrode.
On the first principal surface of the first glass substrate, a first alignment layer 36 is formed or applied so as cover the pixel electrodes and the thin film transistors therewith. Likewise, a second alignment layer is formed or applied on the second principal surface of the second glass substrate so as to cover the common electrodes and the color filters therewith.
In addition, the first and the second glass substrates put a liquid crystal layer between the first and the second alignment layers. The liquid crystal layer includes a plurality of liquid crystal molecules. Each liquid crystal molecule has a long axis and a short axis. The long and short axes of each liquid crystal molecule lies in a plane which is substantially parallel to the surfaces of the first and the second glass substrates and which is defined by both of the vertical direction and a lateral direction. The long axis of each liquid crystal molecule extends in a direction which is called a director in the art. Both of the first and the second alignment layers are subjected to alignment treatment so that the director of each liquid crystal molecule inclines to one side from the vertical direction of the pixel electrodes and the common electrodes by a predetermined angle. This is because there is a chance that a display is poor due to occurrence of domain in the boundaries at which two deformations occur in the opposite direction if the director of each liquid crystal molecule is parallel to the vertical direction. At any rate, the liquid crystal molecules are uniformly aligned upon no generation of the electric field between the pixel electrodes and the common electrodes.
When a lateral electric field generates between the pixel electrode and the common electrode, each liquid crystal molecule rotate in the plane which is substantially parallel to the first and the second principal surfaces of the first and the second glass substrates and so that the director of each liquid crystal molecule is turned in a different direction on no generation of the lateral electric field.
Although the pixel electrodes and the common electrodes are formed on the substrates which are different from each other, JP-A 6-160878 teaches that the pixel electrodes and the common electrodes may be formed on the same substrate.
As described above, the liquid crystal display device of the IPS mode carries out display by rotating each liquid crystal molecule in a plane which is substantially parallel to the surfaces of the substrates caused by generation of the lateral electric field in a plane which is substantially parallel to the surfaces of the substrates. Accordingly, when a person sees or watches the liquid crystal display device from the front, the person""s eyes basically receive light passing through the short axis of each liquid crystal molecule alone although the person moves his or her observing point in a predetermined view range. As a result, the liquid crystal display device of the IPS mode has no dependency on a visual field angle caused by a xe2x80x9crising wayxe2x80x9d of each liquid crystal molecule and it is possible for the liquid crystal display device to achieve a wider viewing angle in comparison with a liquid crystal display device of a twisted nematic (TN) mode. This is because the person""s eyes receive light passing through the long axis as well as the short axis of each liquid crystal molecule in the liquid crystal display device of the TN mode when the person moves his or her observing point in the predetermined view range.
By the way, from a point of view in use conditions or the like, it is necessary for the liquid crystal display device that a contrast ratio related to the viewing angle in the panel right-and-left direction or the lateral direction is symmetrical and a contrast ratio related to the viewing angle in the panel up-and-down direction or the vertical direction is also symmetrical. In other words, it is necessary that the liquid crystal display device has not only a right-to-left symmetrical characteristic of the viewing angle in the right-and-left direction but also a up-and-down symmetrical characteristic of the viewing angle in the panel up-and-down direction. Furthermore, it is desirable that the liquid crystal display device has a symmetrical characteristic of the viewing angle in all of the up-and-down direction and the right-and-left direction. Herein, the contrast ratio (CR) is the ratio of a light transmittance in a state where white is displayed to a light transmittance in a state where black is displayed. In addition, the characteristic of the viewing angle (or an equivalent CR distribution) is a distribution characteristic indicating that how is the contrast ratio distributed for each viewing angle where the person looks in at the panel of the liquid crystal display device.
In the manner which will later be described in conjunction with FIG. 3 in more detail, the characteristic of the viewing angle according to the conventional liquid crystal display device is substantially oval in shape and is neither right-and-left symmetrical nor up-and-down symmetrical. More specifically, in the characteristic of the viewing angle according to the conventional liquid crystal display device, the oval shape of the viewing angle has a major axis which inclines to the right from the panel up-and-down direction by fifteen degrees which are identical with that for the alignment treatment direction of the first and the second alignment layers. This is because the alignment treatment is carried out on the first and the second alignment layers so as to incline to the right from the panel up-and-down direction by fifteen degrees and then the liquid crystal molecules are aligned so that the director of each liquid crystal molecules inclines to the right from the panel up-and-down direction on no generation of the lateral electric field.
As a result, the conventional liquid crystal display device of the IPS mode is disadvantageous in that demands related to the above-mentioned characteristics of the viewing angle are difficult to satisfy.
Other liquid crystal display devices are already known. By way of example, Japanese Unexamined Patent Publication No. Hei 8-190104, namely, JP-A 8-190104 discloses a liquid crystal display element which improves a display grade by suppressing the dependency on a visual field angle or a viewing angle. The liquid crystal display element according to JP-A 8-190104 comprises a first electrode having a plurality of pieces of parallel wire-shaped electrode parts formed on one substrate, a second electrode having a plurality of pieces of parallel wire-shaped electrode parts arranged between the respective wire-shaped electrode parts of the first electrode, and a switching element. Orientation or alignment films have the pretilt angle equal to or less than three degrees and the orientation or alignment direction parallel to the longitudinal direction of the wire-shaped electrode parts. An inter-electrode spacing G and the spacing D between both substrates satisfy the following equation: 0.5xe2x89xa6G/Dxe2x89xa63.0. Inasmuch as this element is not of a type of erect or lay liquid crystals by a change in electric fields, the degradation in transmittance by a difference in the refractive index between the major or long axis direction and minor or short axis direction of the liquid crystals hardly arises. The dependency on the visual field angle is thus suppressed and the display grade is improved.
An electro-optical display device is disclosed in U. S. Pat. No. 5,576,867 issued to Baur et al. The electro-optical display device according to Baur et al. comprises a liquid crystal layer and achieves switching using an electric field component which is predominantly parallel to the surface of the liquid crystal layer. Low dependence of image contrast on viewing angle is achieved by selection of parameters including orientation angle xcex20 and pretilt angle xcex10 of the liquid crystal molecules in the layer. For example, good results are achieved when 0xc2x0 less than xcex20xe2x89xa620xc2x0 for liquid crystal molecules having negative dielectric anisotropy and 70xc2x0xe2x89xa6xe2x88x92xcex20  less than 90xc2x0 for molecules having positive dielectric anisotropy. Preferably, 0xc2x0xe2x89xa6xcex10 less than 30xc2x0.
Japanese Unexamined Patent Publication No. Hei 7-191336, namely, JP-A 7-191336 provides a liquid crystal display device controlled by lateral electric field which has a wide visual field angle, a fast response speed, and superior display control. The liquid crystal display device according to JP-A 7-191336 is provided with a couple of substrates, at least two display electrodes which are formed on one of the substrate while insulated form each other, and a liquid crystal material held between the couple of substrates with the couple of substrates opposed to each other so that the display electrodes are inside. The alignment state of liquid crystal molecules in the liquid crystal material is varied by providing a potential difference between the display electrodes to make a display by controlling rotary polarization performance. This direct view type or projection type liquid crystal display device has the display electrodes formed of inter-digital electrodes having a plurality of teeth, which are partially increased in width into a shape which can control the alignment state so that the alignment state is varied in one direction.
However, in each of the afore-mentioned references, each liquid crystal display device has the characteristic of the viewing angle which is neither right-and-left symmetrical nor up-and-down symmetrical.
It is therefore an object of the present invention to provide a liquid crystal display device which has a characteristic of a viewing angle being right-and-left symmetrical and being up-and-down symmetrical.
Other objects of this invention will become clear as the description proceeds.
In order to resolve the above-mentioned problems, the present inventors made an experiment as regards characteristic of a viewing angle in a case where liquid crystal molecules are not uniformly aligned. As a result, the present inventors disclose that the characteristic of the viewing angle rotates in accordance with a twist angle by twist aligning the liquid crystal molecules by the twist angle using a pair of alignment layers formed on a pair of substrates that are subjected to alignment treatment at alignment treatment angles which are slightly different from each other. On repeating the experiment, the present inventors disclose that it is possible to obtain the characteristic of the viewing angle being right-and-left symmetrical and being up-and-down symmetrical by determining the twist angle in accordance with an average angle between the alignment treatment angles.
Upon examining experimental results statistically, this invention provides, as embodied means for resolving the above-mentioned problems, a liquid crystal display device in the manner which will presently be described.
Specifically, according to an aspect of this invention, a liquid crystal display device comprises first and second substrate having first and second principal surfaces, respectively, which are opposed to each other with a space left therebetween. First and second alignment layers are formed on the first and the second principal surfaces of the first and the second substrate, respectively. The first alignment layer is subjected to a first aligning treatment in a first aligning treatment direction while the second alignment layer is subjected to a second aligning treatment in a second aligning treatment direction which shifts from the first aligning treatment direction by a predetermined slight angle. A liquid crystal layer is interposed between the first and the second alignment layers. The liquid crystal layer comprises a plurality of liquid crystal molecules each of which is twist aligned by a twist angle equal to the predetermined slight angle.